How to Make It
Preheat oven to 425°F. Unroll piecrusts; stack on a lightly floured surface. Roll stacked piecrusts into a 12-inch circle. Fit piecrust into a 10-inch deep-dish, lightly greased tart pan with removable bottom; press into fluted edges. Trim off excess piecrust along edges. Line piecrust with aluminum foil or parchment paper, and fill to rim with pie weights or dried beans. Place pan on a baking sheet. Bake in preheated oven 14 minutes. Remove weights and foil, and bake until golden brown, 10 to 12 more minutes. Reduce oven temperature to 350°F. Cool piecrust completely on baking sheet on a wire rack, about 15 minutes.
Meanwhile, melt butter in a medium skillet over medium-high. Add onions, and cook, stirring occasionally, until tender and lightly browned, about 10 minutes. Stir in asparagus; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Remove from heat, and cool 5 minutes.
Whisk together cream, eggs, chives, parsley, nutmeg, and remaining 1 1/2 teaspoons salt and 1/2 teaspoon pepper.
Place half of onion mixture (about 1 cup) in tart shell; sprinkle with 1/4 cup feta. Spoon half of cream mixture (about 1 1/2 cups) over feta. Repeat layers once, and sprinkle top with remaining 1/4 cup feta.
Bake in preheated oven until set, about 1 hour. Cool on baking sheet on a wire rack 20 minutes before serving.